This invention relates to a single-ended low-pressure discharge lamp, and more particularly to such a lamp which comprises substantially a stem member, an outer envelope with a gas and/or metal vapor filling which is hermetically sealed thereto, two preheatable electrodes arranged within the outer envelope at the stem member and at least one U-shaped inner tube of glass which is also arranged within the outer envelope, with the end of one of the straight limbs or legs of each U-shaped inner tube hermetically surrounding one preheatable electrode and the end of each of the other straight limbs or legs of each inner tube having an aperture communicating with the outer envelope arranged adjacent the stem member. The invention also relates to methods of manufacture of such lamps.
Lamps of such a construction are called compact lamps because they are much shorter than conventional low pressure discharge lamps. When they are equipped with a ballast, starter and screw base, they may be operated in conventional incandescent lamp sockets. With an appropriate construction, such compact lamps are suited as a direct replacement for incandescent lamps. Compact lamps are more economical than incandescent lamps because of their greater efficiency in converting electrical energy into visible light and because of their longer life. In spite of this clear advantage, compact lamps have not yet gained wide acceptance. On the one hand, the proposed constructions are still too complex so that the manufacturing costs rise and the price which the consumer has to pay is higher than the price he is prepared to pay. On the other hand, the light output of lamps of simpler construction is not yet satisfactory. The general aim in the development of compact lamps is to obtain high light output and good color rendition, with the size, shape and price coming as closely as possible to the size, shape and price of the incandescent lamp.
From DE-OS 28 35 574, which corresponds to U.S. Pat. No. 4,199,708, issued Apr. 22, 1980, there is known a single-ended low-pressure discharge lamp which has at least one U-shaped inner tube arranged in the outer envelope. The first end of this inner tube is hermetically sealed about a preheatable electrode, and the other end has an aperture opening to the lamp envelope. The hermetic seal between the U-shaped inner tube and the preheatable electrode to the outer envelope which contains the gas filling is effected by means of an intermediate sealing glass and a chromium iron plate serving as the stem. The soldering of all these components to the intermediate sealing glass requires a complex and thus relatively expensive manufacturing technique. This method of sealing is less suitable to manufacturing large quantities of lamps on fully automatic machines because of the frequently occurring soldering deficiencies.
It is an object of the invention to provide a single-ended low-pressure discharge lamp of the initially described type which as a compact lamp of low power input is comparable in shape and light output to an incandescent lamp and may therefore be used in existing lighting fittings.
A further object of the invention is to permit manufacture of such lamps on high performance machines in order to be able to offer the finished product at a favorable price, and to provide efficient methods of manufacture thereof.